This invention pertains to medical x-ray apparatus, and more particularly to an x-ray image intensifier tube of the proximity type for medical x-ray diagnostic use.
In U.S. Pat. No. 4,140,900 a proximity type image intensifier tube is described. The device uses all linear components and has a high brightness gain. It also has several constructional advantages which contribute to its safety in use, as explained in greater detail in the patent. One disadvantage, however, of the device is that its gain is limited to about 5,000 cd - sec/m.sup.2 - R for high resolution, high contrast applications.
The present applicants have found that many factors contribute to this limitation. Brightness gain in the single stage tube of the type described in U.S. Pat. No. 4,140,900 is proportional to the spacing between the scintillator-photocathode screen and the output phosphor display screen and to the accelerating electrostatic potential applied between them. Wider spacing and higher potential, although producing a higher gain, also reduce the contrast ratio and the resolution.
The reduction in contrast ratio is believed to be due to certain feedback mechanisms operating within the tube. One of these feedback mechanisms is that electrons which strike the output phosphor display screen are, in some cases, reflected back and then are reaccelerated back to the phosphor display screen to strike it again at a different location, thereby reducing both the contrast and the resolution. Also light which is transmitted through the aluminum backing layer on the display screen strikes the photocathode, which produces corresponding electrons, which are then accelerated to strike the phosphor display screen and again reduce the contrast as well as the resolution. Still another feedback mechanism is that because of the high acceleration applied to the electrons traveling from the scintillator-photocathode screen, when the electrons strike the phosphor display screen they produce ions and x-rays which can find their way back to the scintillator and produce unwanted "noise" in the image signal.
Part of the resolution problem is that the scintillator-photocathode surface is relatively rough due to the method (vapor deposition) by which the scintillator material is applied to the support surface. This produces a rough photocathode surface which emits electrons in a relatively wide dispersion. This dispersion is aggrevated as the distance between the scintillator-photocathode screen and the output phosphor display screen is increased.
Originally it was thought that simply increasing the gain would not solve these problems but, on the contrary, would merely aggrevate the problem.